1. Field of the Invention
This invention relates to a method of plating a bonded magnet, and to a bonded magnet carrying a metal coating thereon More particularly, it relates to a method of plating a bonded magnet with a metal coating which is good adhesion has uniform in thickness, has few pinholes, and imparts oxidation and corrosion resistance to the magnet without lowering its magnetic properties, and to a bonded magnet carrying a metal coating of high corrosion resistance on its surface.
2. Description of the Prior Art
Magnets can be broadly classified by their manufacturing process as sintered, cast and bonded magnets, and by their material as alloy magnets made of alloys such as Alnico, Sm-Co and Nd-Fe-B alloys, and oxide magnets made of e.g. ferrites. The sintered magnet is made by compressing a magnetic powder at a high temperature, and the cast magnet by casting a molten metal into a mold. The bonded magnet is made by e.g. the injection, extrusion or compression molding of a mixture of a magnetic powder and a synthetic resin as a binder.
The bonded magnets can be made easily in a wide variety of desired shapes and are, therefore, used for making a wide variety of electrical and machanical parts. They are, however, porous and are, therefore, low in corrosion resistance. After a long time of use, they are likely to have their surface and internal portions oxidized or otherwise corroded, and greatly lower their magnetic properties. It is, therefore, necessary to coat the surface of the bonded magnet in some way or other without lowering its magnetic properties. The bonded magnet is also low at mechanical strength and necessitates the coating of its surface so as not to crack or chip easily. The coating of its surface contributes also to giving it a pleasant or beautiful appearance.
The bonded magnets made of alloys consisting mainly of rare-earth and transition metals (hereinafter referred to as "bonded rare-earth magnets") are used for a particularly wide range of applications owing to their superiority in magnetic properties to the ferrite or Alnico magnets. They have, however, the drawback of being easily oxidized. This is particularly the case with the Nd-Fe-B alloy magnets. The bonded rare-earth magnets undergo a great reduction in magnetic properties as a result of oxidation when used in a highly humid environment, and essentially call for the coating of their surfaces.
Plating is a well-known method which is often used for coating a surface with a metal. There are a variety of methods including vapor deposition, hot dipping, electroless plating, electroplating and substitution plating. Electroless plating has the advantages of being capable of forming a coating having a uniform thickness, coating even the inner surfaces of pores, and being carried out at a low cost by using a simple and inexpensive apparatus. Electroplating has the advantages of being able to form a very adherent coating rapidly at a low cost. Nickel electroplating is particularly beneficial from the standpoints of corrosion resistance and industrial utility.
It is, however, difficult to achieve the desired oxidation and corrosion resistance of a bonded magnet by employing any conventional process for electroless plating or electroplating. Although we, the inventors of this invention, ascertained that the electroplating of a bonded magnet could improve its corrosion resistance to some extent (Japanese Patent Application Laid-Open No. 11704/1991), its corrosion resistance has still been unsatisfactory for any use thereof under harsh conditions, as when it is used for a motor in an automobile.
We made a careful examination of the conventionally electroplated surfaces of bonded magnets, and found that the metal coatings had a by far larger number of pinholes than was macroscopically presumable, and that their corrosion started for the most part in or near the pinholes.
We have studied the reason why such a large number of pinholes are formed in the metal coating on the surface of the bonded magnet. The bonded magnet comprises a magnetic powder and a synthetic resin as a binder, as hereinabove stated, and both the magnetic powder and the synthetic resin are, therefore, exposed in the surface of the magnet. If the magnet is electroplated, the metal used for plating it is first deposited on the exposed magnetic powder, and as the deposited metal grows, it gradually covers the synthetic resin, which is not an electric conductor, until it finally covers the whole surface of the magnet. It is obvious from this process of deposition that the deposited metal forms a coating having a smaller thickness on the exposed synthetic resin than on the magnetic powder. Accordingly, pinholes are more likely to form in the coating on the exposed synthetic resin which is relatively far from the exposed magnetic powder. This is a phenomenon which is peculiar to the bonded magnet, and is apparently due to the difference in electrical resistance from one portion of its surface to another.
The bonded magnet having, for example, a nickel coating formed on its surface by ordinary electroplating is inferior in corrosion resistance to other materials that have likewise been plated. This is due to the fact that the nickel or ammonium chloride, or other chloride that the aqueous solution used for plating contains as the electrolyte penetrates the bonded magnet through its porous surface during its plating, stays in the interface between the magnet and the coating formed thereon, and eventually forms rust, etc. in their interface and the interior of the magnet.
The chloride which the aqueous solution for nickel plating contains promotes the surface activation of the anode and thereby the dissolution of nickel from the anode into the solution, and the removal of the chloride therefrom brings about a great reduction in plating efficiency. Although the application of a high voltage may enable nickel plating in a solution not containing any chloride, the flow of a high electric current to the surface of the material to be plated as a result of its contact with the cathode causes not only the seizure thereof and the formation of a metal coating not having a uniform thickness, but also the heavy polarization of the anode which results in an unstable plating operation. This is particularly the case with a material having a volume resistivity in excess of about 10.sup.-3 ohm-cm, such as a bonded magnet.
Japanese Patent Application Laid-Open No. 42708/1990 discloses an electroplating process which employs an electrolyte composed of an organic solvent and not containing chlorine, as means for overcoming the above problems. The non-aqueous wet plating process which employs an organic solvent has, however, the drawback of being expensive, since the solution which it employs is expensive, and since the apparatus which it employs is complicated and expensive. Thus, there has been a strong desire for a process which employs an aqueous solution and can form a metal coating of good corrosion resistance on the surface of a bonded magnet.
Although electroless plating can be employed for forming a metal coating on the surface of a bonded magnet, it is still difficult to obtain any satisfactory corrosion resistance. This is particularly the case with a bonded magnet made by compression molding which contains a small proportion of a synthetic resin as a binder. It is assumed that an electroless plating solution penetrates a bonded magnet through its porous surface and partly remains in the plated magnet, and that if the solution is acidic or contains chlorine, it corrodes the plated magnet.
When a bonded magnet is plated, it is necessary for its surface to be clean and active, whichever method may be employed for plating it. If its surface is not clean or active, the metal with which it is to be plated fails to adhere closely to its surface. The bonded magnet, however, cannot be said to have a surface which is good for plating, since the oxidation of its surface by heating, the adherence of the binder resin, or a mold release agent to its surface, etc. occur during its manufacture. It is, therefore, usual to cleanse its surface with a strong acid, such as chromic or sulfuric acid, before it is plated. This treatment is, however, not good for, among others, a bonded alloy magnet. The acid not only dissolves and oxidizes the magnetic alloy on its surface and lowers its magnetic properties, but also penetrates the porous surface and interior of the magnet and partly remains in the plated magnet. The remaining acid is very likely to corrode the magnet and impair the adherence of a metal coating to it.